The present invention relates to chemical testing apparatus, and more particularly to spectrophotometric apparatus and method therein for measuring optical density of samples.
The present invention is suited for use in automatic chemical analyzers in which aliquots of a fluid sample to be analyzed are each mixed with selected reagents, depending on the substance whose concentration in the aliquot is being tested. After incubation, the reacted contents of a reaction container, hereinafter referred to as a liquid sample, are measured spectrophotometrically. As used herein, spectrophotometrically may mean a form of measurement in which a radiant energy measurement is a function of the concentration of the substance being tested for in the liquid sample. This may include colorimetric, ultraviolet, fluorescent and nephelometric measurement. Examples of such automatic chemical testing apparatus are found in U.S. Pat. Nos. 3,622,279, 4,039,287 and 4,039,288 all issued to John J. Moran and assigned to the assignee herein, the disclosures of which are incorporated herein by reference.
A common form of measuring optical density of a sample is use of the so called flow cell. Liquid sample is pumped through a cuvette defining a tube. Liquid flows in and out of conduits which are positioned perpendicularly with respect to the tube flowpath. Radiant energy is directed along the length of the flowpath, i.e. along the longitudinal axis of the tube through windows at opposite longitudinal ends of the tube. In this manner, the traditional one centimeter pathlength for spectrophotometric measurements to which the automatic analyzer art has, to a degree, standardized is maintained. Examples of such embodiments are found in U.S. Pat. Nos. 3,241,432 to Skeggs et al and 4,052,161 to Atwood et al. These embodiments have a limited range of optical density which they can measure due to the magnitude of the optical pathlength to a great and optical density in the liquid sample may prevent any radiant energy from reaching a detector. While cuvettes having short optical pahtlengths are known (for example type J48 listed in the catalog of Markson Scientific Supply Co., Inc., Carlsbad, Calif.) they are used for other purposes and are totally unsuited to application in systems in which a number of liquid samples are pumped through the cuvette in succession since intersample contamination would result without an intervening cleaning operation that cannot be readily performed while maintaining a high level of sample throughput.